If you’d like a breakdown of the relative sizes of our planet, our moon and all these other moons, click through for a helpful visual.

Also, there’s a lot of space out there:

You’ll likely need an energy source (not necessarily our star, the warping effects of other massive bodies will do) and tens, if not hundreds, of millions of years to sustain an environment conducive to life.  You’ll likely need lots of protection from cosmic rays and short-wave radiation as some kind of shield.  If your planet, moon and/or body doesn’t possess an atmosphere, and is too small to maintain an electro-magnetic dynamo like Earth, then sub-surface water under a protective shell might be enough.

On the recent findings of at least 1/17 observational days of water plumes near the surface of the Jovian moon, Europa:

More here:

‘They used a spectrograph at the Keck Observatory that measures the chemical composition of planetary atmospheres through the infrared light they emit or absorb. Molecules such as water emit specific frequencies of infrared light as they interact with solar radiation.’

and:

‘Paganini and his team reported in the journal Nature Astronomy on November 18 that they detected enough water releasing from Europa (5,202 pounds, or 2,360 kilograms, per second) to fill an Olympic-size swimming pool within minutes. Yet, the scientists also found that the water appears infrequently, at least in amounts large enough to detect from Earth, said Paganini: “For me, the interesting thing about this work is not only the first direct detection of water above Europa, but also the lack thereof within the limits of our detection method.’

This is potentially good news for the upcoming Europa clipper mission.  Otherwise, how are you going to get at all that water beneath an icy shell at least 10-15 miles thick?

‘From its orbit of Jupiter, Europa Clipper will sail close by the moon in rapid, low-altitude flybys. If plumes are indeed spewing vapor from Europa’s ocean or subsurface lakes, Europa Clipper could sample the frozen liquid and dust particles. The mission team is gearing up now to look at potential orbital paths, and the new research will play into those discussions.

“If plumes exist, and we can directly sample what’s coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life,” Pappalardo said. “That’s what the mission is after. That’s the big picture.”

Aren’t you getting a little excited at the prospect?: